The biological clock ticks not just for women, suggests new research that may overturn conventional thinking about mens virility as they age.

Whereas women undergo menopause as a clear indicator of decreased fertility, a similar biological mechanism in men was previously unknown. But new research into delivery rates among couples undergoing in vitro fertilization found that men hear the clock ticking, too.

The study, assessing nearly 19,000 cycles of in vitro fertilization, showed that, as a mans age increased, the success rate of fertilization went in the opposite direction.

Generally, we saw no significant decline in cumulative live birth when women had a male partner the same age or younger, said study author Dr. Laura Dodge, researcher with the Beth Israel Deaconess Medical Center and Harvard Medical School, who presented her findings at the 33rd annual meeting of the European Society of Human Reproduction and Embryology (ESHRE) in July.

But the success rate changed when looking at men of different ages, with younger men conferring a significant fertility benefit to their partners.

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Women aged 35-40 did significantly benefit from having a male partner who is under age 30, in that they see a nearly 30% relative improvement in cumulative incidence of live birth when compared to women whose partner is 30-35 from 54% to 70%, said Dodge.

That trend reverses when women in that age group are paired with similar-aged men.

Where we see significant decreases in the cumulative incidence of live birth is among women with male partners in the older age bands. For women age 30-35, having a partner who is older than they are is associated with approximately 11% relative decreases in cumulative incidence of live birth from 70% to 64% when compared to having a male partner within their same age band, explained Dodge.

While the study results add some nuance to the question of mens impact on fertility, the researchers note that women still add the most to the mix.

Both the results of this study and prior work show that female age has a larger effect on fertility than male age. While the effect of female age on fertility is overwhelmingly due to increased rates of chromosomal abnormality, the proposed mechanisms in the effect of male age on pregnancy are more subtle, said Dodge.

When we looked at the effect of female age alone, we saw a 46% relative decrease from ages under 30 to 40-42, but when we looked at male age alone, we saw a 20% relative decrease over the same age span, she added.

The study raises questions about what, if anything, aging men can do to boost conception rates when pairing with younger or same-aged women.

Its hard to say without knowing the precise mechanisms involved, said Dodge. Most preconception advice for men focuses on semen quality, though studies suggest that this likely cannot fully ameliorate the effects of male reproductive ageing. So in the absence of clear evidence of the mechanisms, the best preconception advice we can offer is to maintain a healthy lifestyle.

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Richard Scott is a health care reporter focusing on health policy and public health. Richard keeps tabs on national health trends from his Philadelphia location and is an active member of the Association of Health Care Journalists.

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